I have a new dell t105(opty 1210's with 4 gigs of ram..sata hdd's and sata dvd-rom). I can bot to the Cent 5 64 bit dvd fine but once it asks where the install image is i tell it cd-rom..then it rejects the dvd spits it out and tells me it's not there. the same happens on 32 bit. I have downloaded centos5 twice now, md5 verified..and even burned at 1x(that took forever). This system does not have any ide ports...it is sata only. The cd images work just fine..but the dvd iso's do not.

I run into the exact same issue installing on a home build server with the following HW configuration:
Processor: Core 2 duo
DVD Drive: External USB
HD: 2x 80GB ATA; 5x 320GB SATA
The system can boot to the DVD disk but failed to find it at the select source step.

I do not have the virtual console set up to capture
the output. I have telnet into the console port(ttyS0),
but the install process does not go through ttyS0, so
I can't capture the output.

On my 32 and 64 bit du-core system. The disk or volume
have been partitioned and have Red Hat 5 installed on
one or two partiotons. It looks like only the option one
that remove all partitions and install default
configuration work for both 32 and 64 bit
iso image dvd. I tried to choose the option 2 and 3
to select a unused partition or create a new partition
to install CentOS 5 so I can preserve my Red Hat 5, they
both failed.
On my 32 bit system, I have two disks. So I can install
CentOS on my second disk. On my 64 bit system, I only have
one big volume, so I end up wiping out my Red Hat 5 system.

I have the same or very similar problem with a Dell power edge R200.
I have two similar machines one has 1GB of ram + 1x250G HD + 1 DVD drive (TSSTCorp) The other machine has 2GB ram, 2x250G HD + 1 DVD.
All drives are SATA.
The machine with one HD installed fine.
The machine with 2 HD's will not install. The installer can not find the DVD drive.
The drives are arranged such that the HD's are on the first two channels and the DVD on the third.
The machine that did install with only one HD had the DVD on the second channel.

I suspect that the installer is not searching for the DVD after the second channel.

Unfortunately I am unable to capture the virtual consoles, I can't even read them all in time to confirm my suspicions. Is there a trick to capturing the virtual consoles? If so I'll do it and post here.

As asked before: Can anyone of you take a look at the virtual consoles if there is an error message which could explain this problem? You can reach those with <ALT>-<F2> through <F6> if you are still in "text mode", <CTRL>-<ALT>-<F1> through <F6> in graphical mode (at least I think F6 is the last console).

I had the same problem. I have hit enter on the start screen [to boot into graphical interface], but it started text interface after medium check... The system in text mode was unable to detect DVD as source medium...

On cca fifth try [only hitting enter on start up - nothing more ;) ], I have sucessfully booted to graphical interface and from there the installation was going OK...

After the first booting into graphical interface, I tried to repeat the error and see the virtual consoles for error messages, but second booting was correct into the graphical interface, i will try to experiment with this...

Apparently the sata_nv driver in CentOS kernel doesn't support whatever SATA optical drive Dell installs in this server. I have tried "linux sata_nv.adma=0" in the boot prompt, since Dell suggests it as a workaround to install SUSE. I quote:

----------------
SUSEÂ® Linux Enterprise Server 10 Service Pack 1 does not support internal
SATA optical drives on systems with more than 4 GB of system memory.
The SATA optical drive connected to the SATA controller is not recognized
during and after installation because of an issue with the sata_nv driver. A fix
for this issue will be available in a future service pack.
To work around this issue, you must load the sata_nv driver with the adma
mode disabled during boot-up.
----------------

I have exactly 4 GB of memory. Anyway, this workaround doesn't help CentOS in the slightest. Even if I manage to network install it the optical drive is not usable. Any attempt to mount /dev/scd0 results in a crash and automatic reboot of the system!

I was able to get into the graphics installation UI with the new 5.2 x86_64 DVD after media check. I did not perform the actual install since I do not want to replace my running system but I went all the way to right before the "click next to begin upgrade of CentOS" prompt. I think this version is working.

workaround provided citing the reporter :)
...
Booting with mem=3G did indeed work around the bug!!!
Curiously enough, I had read of this workaround and tried it for the
DVD install. mem=3G created a kernel panic about not finding the root
partition, so I desisted of exploring that avenue.

However this option works for the installer if you express it in MB
instead of GB, i.e., mem=2048M will allow you to perform a DVD
install. mem=2G won't work for the installer, but it is ok once the
system is already installed.
...

The DVD install runs fine when I add the mem=2048M option to workaround the memory issue. I'm not setup to test the kernel patch, but the description certainly sounds like it would fix the problem. Thank you Luis for tracking this down to the root cause.